Pediculus humanus humanus (the body louse) is indistinguishable in appearance from Pediculus humanus capitis (the head louse) but will interbreed only under laboratory conditions. In their natural state, they occupy different habitats and do not usually meet. In particular, body lice have evolved to attach their eggs to clothes, whereas head lice attach their eggs to the base of hairs.
The life cycle of the body louse consists of three stages: egg (also called a nit), nymph, and adult.
Nits are louse eggs. They are generally easy to see in the seams of an infested person's clothing, particularly around the waistline, under armpits or even in body hair. They are oval and usually yellow to white in color. Body lice nits may take 1-2 weeks to hatch.
A nymph is an immature louse that hatches from the nit (egg). A nymph looks like an adult body louse, but is smaller. Nymphs mature into adults about 9-12 days after hatching. To live, it must feed on blood.
The adult body louse is about the size of a sesame seed (2.5-3.5 mm), has six legs, and is tan to greyish-white. Females lay eggs. To live, lice must feed on blood. If separated from their hosts, lice die.
The body louse diverged from the head louse at around 100,000 years ago, hinting at the time of the origin of clothing. Body lice were first described by Carl Linnaeus in the 10th edition of Systema Naturae. The human body louse had its genome sequenced in 2010, and at that time it had the smallest known insect genome. Other lice that infest humans are the head louse and the crab louse. The claws of these three species are adapted to attachment to specific hair diameters.